Inhalation anesthetics are volatile substances with relatively low boiling points and high vapor pressures. Such anesthetics are typically dispensed in liquid form to an apparatus, such as a vaporizer on an anesthesia machine, which mixes the anesthetic with oxygen and nitrous oxide. The mixture is supplied by the machine in gaseous form to the patient for inhalation.
During a typical inhalation anesthetic procedure, only a small amount of the active agent or anesthetic is taken up by the patient. For example, when a patient inhales, some amount of the anesthetic enters the lungs, but upon exhalation, up to about 99% of the inhaled anesthetic is exhaled.
In some anesthesia machines, the exhaled breath is exhausted and cannot be recirculated to the patient. However, improved types of machines have been designed to recirculate the exhausted anesthetic in order to reduce waste and expense. Such machines permit the exhaled breath to be purged of carbon dioxide (CO.sub.2), blended with an appropriate amount of fresh anesthetic and gas, and recirculated to the patient.
Anesthesia machines which recirculate the anesthetic along with the patient's exhaled breath typically employ a soda lime scrubber to remove carbon dioxide. Soda lime typically contains from about 3 percent to about 5 percent sodium hydroxide/potassium hydroxide and approximately 20 percent water, which, in the presence of carbon dioxide, react to form carbonate species. This effectively removes most of the carbon dioxide from the gas stream. Heat is produced in this process. The observed heat generation in the soda lime scrubber is thought to be due to the exothermic, carbonate-forming chemical reactions as well as to the exothermic dissolution of the soda lime constituents. The heat produced by the exothermic reactions increases the reaction rates. In this specification it shall be understood that the term "exothermic reaction" includes a chemical reaction as well as a physical reaction (e.g., dissolution) of the type that produces or generates heat.
While such scrubbing systems generally function satisfactorily, there are potential problems that may arise when using certain, newer anesthetics. For example, a new anesthetic which may in the future be approved for use in the U.S.A. is a fluoromethyl 2,2,2-trifluoro-1-(trifluoromethyl)ethyl ether sold under the trademark SEVOFLURANE.TM. and licensed by Abbott Laboratories, Inc., One Abbott Park Road, Abbott Park, Ill. 60064-3500 U.S.A.
When SEVOFLURANE.TM. anesthetic in the patient's exhaled breath passes through a soda lime or similar type carbon dioxide scrubber, the anesthetic is exposed to the heat generated by the above-discussed exothermic reactions or other heat generating processes. Further, if the system is exposed to abnormal, higher temperature operating or ambient conditions, then there could be additional heat transfer to the anesthetic.
Regardless of the source or sources of the heat, the anesthetic might then suffer degradation from exposure to heat in the presence of soda lime, and a degradation by-product could be produced. Even at normal operating conditions in the scrubber of an anesthesia machine, the concentration of such an anesthetic and the gas flow rates are such that some degradation of the anesthetic occurs as a result of the heat produced by the above-described carbon dioxide scrubbing process. In order to eliminate or minimize the potential for such degradation of the anesthetic, and in order to operate with a greater safety margin, it would be desirable to provide an improved method and apparatus for controlling system temperatures.
The present invention provides an apparatus and method having the above-discussed benefits and features.